1. Field of the Invention
The present invention relates to a color registration deviation correction method for a laser beam printer, a digital copying machine, or the like in which a laser beam is scanned by an optical scanning device provided with a rotating polygon mirror and a multicolor image is formed by an electrophotography system, and an image forming apparatus, and particularly to a color registration deviation correction method having a function of correcting a color registration deviation as a position deviation between respective colors in a sub-scanning direction, and an image forming apparatus.
2. Description of the Related Art
In a multicolor image forming apparatus using an electrophotography system, toners of four colors of black (K), cyan (C), magenta (M), and yellow (Y) are superimposed on a sheet and are transferred, so that multicolor printing is performed. When multicolor printing is performed, if writing positions of the respective colors subtly deviate from each other, a color deviation appears when they are superimposed and transferred on the sheet, and printing quality is degraded. Then, in order to solve this and to obtain a multicolor print of high precision, it becomes necessary to perform writing position control for correcting the color registration deviation.
As a correction of the color registration deviation, there has been conventionally proposed a correction method in which a toner image of a specified pattern is formed on an intermediate transfer body or the like, the pattern is detected by an optical sensor including an optical source, a photodetector, and the like, a color registration deviation amount is calculated, data of a correction amount is transmitted to each correcting system, and correction is made, or a correction method in which a specified pattern is previously prepared in a region outside of an image formation region of an intermediate transfer body or the like, the pattern is detected by a sensor, a color registration deviation amount is calculated from the detection timing of the pattern and the detection timing of a writing timing signal (SOS signal) or the like in a main scanning direction, data of a correction amount is transmitted to each correcting system, and correction is made.
Among various kinds of corrections, especially in the case where writing position correction in a sub-scanning direction is made, high precision correction can be made by combining a section of correcting a writing position in a unit of one line and a section of correcting a writing position in a unit of less than one line. As shown in FIG. 12A, a print start signal (image forming signal) is made a trigger, a predetermined number of writing timing signals (SOS signals) in the main scanning direction is counted, and writing timings of the respective colors are made coincident with each other with high precision, so that images are superimposed. Here, if a normal writing timing is made (2) in FIG. 12A, when the count number of the SOS signals is increased or decreased ((1) and (2) in FIG. 12A), as shown in FIG. 12B, the writing position is moved up and down in a unit of one line. By using this, the writing timing can be controlled in a unit of one line, and writing position correction can be made.
As a method of correcting a writing position in a unit of less than one line, as shown in FIG. 13C, a rotation phase of a rotating polygon mirror (so-called polygon mirror) is changed so that the correction can be made. It is assumed that an output timing of a normal SOS signal is made (2) in FIG. 13A. When the rotation phase of the polygon mirror is changed ((1) and (2) in FIG. 13C), as shown in FIG. 13A, the generation timing of the SOS signal is changed in accordance with this ((1) and (2) in FIG. 13A), and as shown in FIG. 13B, the writing position in the sub-scanning direction is moved up and down. The rotary phase of the polygon mirror is controlled within the range where the movement of the writing position becomes less than one line, so that the correction can be made.
The polygon mirror is attached to a polygon mirror driving motor. As shown in FIG. 14A, PLL (Phase Locked Loop) control is performed by a motor (MOT) control circuit with a reference clock and a rotation frequency signal (FG output or hole element output in a polygon mirror driving motor, or scanning light position detector output by a photodetector arranged outside of a scan surface in an optical scanning device, which becomes a writing position timing signal in the main scanning direction) such as a SOS signal, so that the polygon mirror driving motor rotates at constant speed of rotation with high precision.
As shown in FIG. 14B, control of the polygon mirror rotation phase can be made by controlling the phase of the reference clock given to the polygon mirror driving motor for rotating the polygon mirror. That is, the phase of the reference clock is changed, so that a phase difference is generated between the rotation frequency signal and the reference clock, and the rotation speed of the polygon mirror driving motor is changed so as to remove this. With this, the rotation phase of the polygon mirror is also changed. Then, the rotation phase of the polygon mirror is changed, so that the output timing of the writing timing signal in the main scanning direction is also changed with this. Since the writing timing in the main scanning direction is kept constant in accordance with the writing timing signal in the main scanning direction, the writing timing in the sub-scanning direction can be controlled.
As such a correction technique of color registration deviation, especially as a correction technique of less than one line, there is a technique disclosed, for example, Japanese Patent Unexamined Publication No. Hei. 8-152833.
According to the technique disclosed in Japanese Patent Unexamined Publication No. Hei. 8-152833, in an apparatus which includes one image forming apparatus and in which a multicolor image is formed by superimposing plural toner images on an intermediate transfer body, a phase matching period in which a frequency of a reference clock given to a polygon mirror driving motor at the time of rotation phase control of a polygon mirror is made slightly low or high is provided, and the frequency is returned to the original one after the phase matching period corresponding to a control amount, so that the phase of a mirror surface of the polygon mirror is gradually delayed or advanced, and control is made to obtain a desired phase (see FIGS. 15A and 15B). By this, writing timing in the sub-scanning direction is controlled, and color registration position deviation in the sub-scanning direction is corrected, which is proposed in this publication.
Besides, there is also such a technique that in an apparatus in which a multicolor image is formed by plural image forming devices and by sheet conveyance at one time, plural master clocks as references of phase control are previously provided, and when a color registration deviation in a sub-scanning direction is detected, a clock corresponding to a correction amount is selected among the plural master clocks, and further, the frequency of the reference clock given to a polygon mirror driving motor is made slightly low or high, and the frequency is returned to the original one at the point of time when the reference clock and the master clock are coincident with each other, so that writing timing in the sub-scanning direction is controlled and the color registration deviation in the sub-scanning direction is corrected in less than one line (see FIGS. 14A, 14B, and 16A).
In recent years, in an image forming apparatus such as a digital copying machine or a laser beam printer, high speed and high resolution are demanded. Besides, as described above, the technique of color registration deviation correction is indispensable to a multicolor image forming apparatus to keep a high quality image. In order to keep the high quality image and make printing at high speed, a technique to control color registration during continuous printing without interruption of a print job becomes necessary.
This technique is such that a specified pattern of each color is formed of a toner image in a period (hereinafter referred to as an interimage period) between images formed on a primary transfer part such as an intermediate transfer body, the pattern is detected by an optical sensor including a light source, a photodetector, and the like, and a color registration deviation amount is calculated, so that correction is made in the inter image period different from pattern formation, or such that a detection timing of a specified pattern previously prepared in a region outside of an image formation region of an intermediate transfer body or the like and an output timing of a writing timing signal or the like in the main scanning direction are detected in a period of an each single color image formation interval, and writing timing in the sub-scanning direction is changed. The shorter the interimage period is, the faster the print performance of the apparatus becomes.
However, in the conventional multicolor image forming apparatus including plural image forming devices, if the interimage period during the print job in the continuous printing is made short, there occur problems as set forth below.
In Japanese Patent Unexamined Publication No. Hei. 8-152833 and in the technique of using the foregoing plural master clocks, the frequency is finely increased (here, an explanation will be made on a system in which the phase is advanced) by the structure as shown in FIGS. 15A and 14A to gradually change the phase, so that occurrence of rapid rotation change is prevented, and a time (stabilization time) when the polygon mirror returns to the original revolution speed can also be shortened. However, as a control amount becomes large, a control time becomes long, and as shown in FIGS. 15C and 16B, a time xcex94t (stabilization time) from the occurrence of the rotation change to the return to the original revolution speed becomes very long as compared with a frequency changing time.
For example, when the control amount is large (see FIG. 17), if printing is made on a next sheet before the rotation of the polygon mirror driving motor is stabilized, as shown in FIG. 17B, the influence of rotation variation appears on the image and becomes a color deviation to damage the print quality. Thus, it is necessary to convey the next sheet after a sufficient time has elapsed until the rotation of the polygon mirror driving motor becomes stable, so that the output capacity of the print image (speed of print processing) can not be raised.
Besides, when the change of the rotation phase of the polygon mirror is continuously carried out, there is a case where the phase of an SOS signal after each correction becomes equal to or more than the phase of an SOS signal before one line or after one line of the reference color. For example, as shown in FIGS. 18A to 18E, in the correction of FIGS. 18A to 18C, since the phase of the SOS signal after the correction does not exceed the phase of the SOS signal one line before the reference color, the correction can be made without any problem. However, as in FIGS. 18D to 18E, in the case where the phase of the SOS signal after correction exceeds the phase of the SOS signal one line before the reference color, there is a problem that the count start timing of the SOS signal with the print start signal as a trigger is deviated, and by this, the writing start timing in sub-scanning is also moved by one line, so that the color registration deviation of one line in sub-scanning appears on the sheet.
The present invention has been made in view of the above circumstances and provides a color registration deviation correction method and an image forming apparatus in which an image can be formed while a high quality image is kept and color registration can be controlled without interrupting image formation.
According to an aspect of the present invention, a color registration deviation correction method of an image forming apparatus which forms an image by light scanning of a rotating polygon mirror reflecting and deflecting light from a light source, includes the steps of detecting a color registration deviation; calculating, from the detected color registration deviation, a correction value for making a correction of the color registration deviation; and dividing the calculated correction value to make the color registration deviation correction plural times.
According to another aspect of the present invention, a color registration deviation correction method of an image forming apparatus which forms an image by light scanning of a rotating polygon mirror reflecting and deflecting light from a light source, includes a first step of detecting a color registration deviation and calculating, from the detected color registration deviation, a correction value for making a correction of the color registration deviation; a second step of judging, on the basis of the correction value calculated at the first step, whether a time needed to make the color registration deviation correction is longer than a predetermined period; and a third step of correcting the color registration deviation in the predetermined period when it is judged that the time is shorter than the predetermined period at the second step, and making the color registration deviation correction plural times when it is judged that the time is longer than the predetermined period at the second step.
According to the above invention, in the first step, the color registration deviation is detected, and the correction value for making the correction of the color registration deviation is calculated. In the second step, on the basis of the correction value calculated at the first step, it is judged whether the time needed to make the color registration deviation correction is longer than the predetermined period.
In the third step, the color registration deviation correction is completed in the predetermined period when it is judged that the time need to make the color registration deviation correction is shorter than the predetermined period at the second step, and the color registration deviation correction is divided and is made plural times when it is judged that the time need to make the color registration deviation correction is longer than the predetermined period at the second step. That is, in the case where the correction of the color registration deviation amount is not completed in the predetermined period, the color registration deviation correction is made plural times, for example, twice. Thus, it is possible to prevent such a case that the color registration deviation correction overlaps with the time of image formation so that a bad influence is exerted on the image, and it is possible to certainly correct the color registration deviation.
Accordingly, the image can be formed while the high quality image is kept, and the color registration can be controlled without interrupting the image formation.
According to another aspect of the present invention, an image forming apparatus which forms an image by light scanning of a rotating polygon mirror reflecting and deflecting light from a light source, includes a detection unit which detect a color registration deviation; a calculation unit which, from the color registration deviation detected by the detection unit, calculates a correction value for correcting the color registration deviation; and a control unit which divides the correction value calculated by the calculation unit and causes the color registration deviation correction to be made plural times.
According to another aspect of the present invention, an image forming apparatus which forms an image by light scanning of a rotating polygon mirror reflecting and deflecting light from a light source, includes a detection unit which detects a color registration deviation; a calculation unit which calculates, from the color registration deviation detected by the detection unit, a correction value for correcting the color registration deviation; and a control unit which causes the color registration deviation to be corrected in a predetermined period when a time need to make a correction of the color registration deviation on the basis of the correction value calculated by the calculation unit is shorter than the predetermined period, and causes the color registration deviation correction to be made plural times when the time is longer than the predetermined period.
According to the above invention, the detection unit detects the color registration deviation, and the calculation unit calculates, from the detected color registration deviation, the correction value for correcting the color registration deviation. The control unit causes the color registration deviation to be corrected in the predetermined period when the time need to make the correction of the color registration deviation on the basis of the correction value calculated by the calculation unit is shorter than the predetermined period (for example, the period between image formation and image formation, or the like), and causes the color registration deviation correction to be made plural times when the time need to make the correction of the color registration deviation is longer than the predetermined period. That is, in the case where the color registration deviation amount is not corrected in the predetermined period such as the period between the image formation and the image formation, the color registration deviation correction is made plural times. Thus, the color registration deviation can be certainly corrected without exerting a bad influence to the image.
Accordingly, the image can be formed at high speed while the high quality image is kept, and the color registration can be controlled without interrupting the image formation.
Incidentally, the plural corrections can be determined in accordance with the color registration deviation amount which can be corrected in the predetermined period.